Flat-panel displays, such as light emitting diode (LED) displays, of various sizes are proposed for use in many computing and communication applications. In its simplest form, an LED includes an anode for hole injection, a cathode for electron injection, and a light-emitting medium sandwiched between these electrodes to support charge recombination that yields emission of light. LED displays can be constructed to emit light through a transparent substrate (commonly referred to as a bottom-emitting display), or through a transparent top electrode on the top of the display (commonly referred to as a top-emitting display). Both organic and inorganic light-emitting materials are known and may be formed into thin-film layers.
Full-color displays employing light-emissive materials are known in the art. Typical full-color displays are constructed of three different color pixels that are red, green, and blue in color. Such an arrangement is known as an RGB design. An example of an RGB design is disclosed in U.S. Pat. No. 6,281,634. One of the main challenges of manufacturing full-color displays is the patterning of light-emissive materials. For evaporated organic materials, precision shadow mask technology is most commonly used today in manufacturing. Although shadow mask deposition of organic LED materials can work on a substrate of moderate size, e.g., 300 mm×400 mm, it becomes difficult with larger substrates or when the pixel density becomes very high, such as in top-emitting displays. One problem is the handling (fabrication, alignment, etc.) of such large, thin, and fragile shadow masks. Another problem is the thermal coefficient of expansion mismatch between the shadow mask, through which the organic LEDs are deposited, and the underlying substrate. This leads to misalignment of the mask and the proper deposition area on the substrate. Furthermore, this technique is not useful for patterning materials that are not readily evaporated.
Another challenge to top-emitting LED devices is that a transmissive top electrode is typically provided as a common electrode for many or all pixels. Unfortunately, the most effective transmissive electrode materials, e.g., ITO and other metal oxides, have insufficient conductivity across the substrate, especially for large substrates. One way to solve this problem is to introduce a highly conductive auxiliary electrode or bus. Numerous bussing designs have been proposed, e.g., in U.S. Published Patent Application Nos. 2004/0253756; 2002/0011783 and 2002/0158835, but such designs add additional complexity to the manufacturing process.
Semiconductor light-emitting diode (LED) devices, which are primarily inorganic, have been made since the early 1960's and currently are manufactured for usage in a wide range of consumer and commercial applications. The layers comprising the LEDs are based on crystalline semiconductor materials. These crystalline-based inorganic LEDs have the advantages of high brightness, long lifetimes, and good environmental stability. The crystalline semiconductor layers that provide these advantages also have a number of disadvantages. The dominant ones have high manufacturing costs; difficulty in combining multi-color output from the same chip; efficiency of light output; and the need for high-cost rigid substrates. However, in comparison to OLEDs, crystalline-based inorganic LEDs have improved brightness, longer lifetimes, and do not require expensive encapsulation for device operation.
Quantum dots are light-emitting nano-sized semiconductor crystals. Adding quantum dots to an organic emitter layer enhances the color gamut of the device; red, green, and blue emission is obtained by simply varying the quantum dot particle size; and manufacturing costs are reduced. Because of problems such as aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., Journal of Applied Physics 93, 3509-3514 (2003)). The poor efficiency was attributed to the insulating nature of the quantum dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a mono-layer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800-803 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the organic molecules (electron-hole recombination occurs on the organic molecules).
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039-1044 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum dot layer between vacuum deposited inorganic n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) that were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection onto the quantum dots. In addition, the remainder of the structure is costly to manufacture, due to the usage of electron and hole semiconducting layers grown by high-vacuum techniques, and the usage of sapphire substrates.
As described in co-pending, commonly assigned U.S. patent application Ser. No. 11/226,622, filed Sep. 14, 2005 by Kahen, which is hereby incorporated by reference in its entirety, additional conducting or semi-conducting particles may be provided with the quantum dots in a layer to enhance the conductivity of the light-emitting layer.
Light-emitting diode structures may be employed to form flat-panel displays. Likewise, colored-light or white-light lighting applications are of interest. Different materials may be employed to emit different colors and the materials may be patterned over a surface to form full-color pixels. In various embodiments, the quantum dot LEDs may be electronically or photonically stimulated and may be mixed or blended with a light-emitting organic host material and located between two electrodes.
A prior-art structure employing electronic stimulation uses a substrate on which is formed a first electrode, a light-emissive layer, and a second electrode. Upon the application of a current from the electrodes, electrons and holes injected into the matrix create excitors that are transferred to the quantum dots for recombination, thereby stimulating the quantum dots to produce light. Such a design is described in WO 2005/055330 A1 entitled “Electroluminescent Device.” P-type and/or an n-type organic transport, charge injection, and/or charge blocking layers may be optionally employed to improve the efficiency of the device. Typically, one electrode will be reflective while the other may be transparent. No particular order is assumed for the electrodes.
A typical LED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of charge-control and light-emitting layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate, and thus is commonly referred to as a bottom-emitting device. Alternatively, an LED device can include a substrate, a reflective anode, a stack of charge-control and light-emitting layers, and a top transparent cathode layer. Light generated from this alternative device is emitted through the top transparent electrode, and thus is commonly referred to as a top-emitting device. In general, bottom-emitting LED devices are easier to manufacture, because the transparent electrode (e.g. ITO) employed in a top-emitting device may be difficult to deposit over the charge-control and light-emitting layers without damaging them and suffers from limited conductivity. In contrast, the evaporation of a reflective metal electrode has proved to be relatively robust and conductive. However, active-matrix bottom-emitting LED devices suffer from a reduced light-emitting area (aperture ratio), since a significant proportion (over 70%) of the substrate area can be taken up by the active-matrix components, bus lines, etc. Since some LED materials degrade in proportion to the current density passed through them, a reduced aperture ratio will increase the current density through the layers at a constant brightness, thereby significantly reducing the LED device's lifetime. Top-emitting LED devices can employ an increased aperture ratio, since light emitted from the device passes through the cover, rather than the substrate. Active-matrix devices formed on the substrate can be covered with an insulating layer and a reflective electrode formed over the active-matrix components, thereby increasing the light-emitting area. Active-matrix components, typically thin-film transistors are formed on the substrate using photolithographic processes.
Thin-film, LED devices in general suffer from a loss of light trapped in various layers of the LED, substrate, or cover, thereby decreasing the efficiency of the LED device. Typical indices of refraction for charge-control and light-emitting layers range from 1.6 to 1.7 for organic materials and well over 2.0 for inorganic layers and the refractive index of commonly used transparent conductive metal oxides, such as indium tin oxide (ITO) is often greater than 1.8 and often near 2.0. Hence, light emitted in a layer at a high angle with respect to the substrate normal can internally reflect and become trapped in the high optical-index materials of the layers and transparent electrodes; thereby reducing the efficiency of the LED device.
Because light may be emitted in all directions from the internal organic layers of the LED, some of the light may be emitted directly from the device, while some light is emitted into the device and either absorbed or reflected back out. Some of the light may be emitted laterally, or trapped and absorbed by the various layers comprising the device. Light generated from an LED device can be emitted through a top transparent electrode comprised of ITO, but it has been estimated that only about 20% of the generated light is actually emitted from such a device. The remaining light is trapped by internal reflections between layers and eventually absorbed.
Scattering techniques are known to improve the efficiency of light emission from an organic LED device. Chou (International Publication Number WO 02/37580) and Liu et al. (U.S. Patent Application Publication No. 2001/0026124) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light produced within the organic LED device at higher than the critical angle, which would have otherwise been trapped, can penetrate the scattering layer and be scattered out of the device. The efficiency of the organic LED device is thereby improved. However, scattered light can propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixelated applications such as displays. Scattering techniques cause light to pass through the light-absorbing material layers multiple times where they can be absorbed and converted to heat.
Therefore, a need exists to provide more effective ways to employ optical materials, such as color filters, light scattering materials, and auxiliary electrodes in LED display formats.